The present invention relates to an light source to output ASE light by supplying pumping light to an optical amplifying fiber.
An optical fiber doped with rare earth ions emits light in an amplifying signal band when the ion excited to a high energy level by pumping light returns to the stable ground level. The emitted light is referred to as amplified spontaneous emission light (ASE light). ASE light sources are used to emit the ASE light as light sources. For example, such an ASE light source can be used to determine wavelength loss characteristics of an optical device by connecting to the device which is coupled to an optical spectral analyzer or other measuring instrument.
FIG. 13 shows a conventional ASE light source composed of an amplifying rare-earth doped optical fiber 3 such as Er-doped fiber, a pumping source 6, and an optical isolator 1. In this ASE light source, erbium ions doped in the amplifying optical fiber 3 are excited into a high energy level by pumping the light from the pumping source 6, and then, emits ASE light in the wavelength band individual to the ion when the excited energy higher than the ground level is emitted. The isolator 1 prevents the ion excitation in the amplifier fiber from being made unstable by the returning light from the output end of the fiber
While the emitted ASE light from Er-doped amplifying fiber amplifier typically includes a wavelength band of 1530 to 1570 nm (1550 nm band), the fiber can emit a wavelength band of 1570 to 1610 nm as the fiber length is elongated 4 to 6 times as long as the usual fiber length. See Ono et al; xe2x80x9cAmplifying Characteristics of 1.58 xcexcm Band Er3+-Doped Optical Fibers Amplifierxe2x80x9d, Technical Report of Institute Of Electronics, Information and Communication Engineers, Japan, No. 5, pp 25-29, 1997.
In addition, 36 nm of a half-width of the ASE light has been achieved by a quartz Er-doped fiber (in a range of 1567 to 1604 nm), and 40 nm (1563 to 1603 nm) by a fluoride Er-doped fiber.
FIG. 14 shows the determined wavelength distribution of ASE power from the Er-doped fiber 3 with a variable fiber length. The data were determined at a pumping wavelength of 1480 nm (the 1480 nm pumping having a higher pumping efficiency with respect to amplified spontaneous emission than the 980 nm pumping), with pumping power of 100 mW, and fiber lengths varied between 20 m and 150 m. It has been seen from FIG. 14 that the length of the fiber shifts the amplified wavelengths from the 1550 nm band to the 1580 nm band, while an excessive length of the fiber lowers the output power from the ASE light sources.
Also, it in known that an Er doped tellurite fiber provides a signal amplifying performance in a broader band, for example, in a range of 1530 to 1610 nm, than conventional. See M. Yamada et al; xe2x80x9cLow -Noise Gain-Flattened Er3+Doped Tellurite Fiber Amplifierxe2x80x9d, optical Amplifiers and Their Applications, 1998, Technical Digest, pp 86-89 (1998), Optical Society of America. According to the above, tellurite fibers which are formed by doping erbium in tellurite glass fibers, because of relatively low reliability to moisture, must, in use, be sealed perfectly with particular air-tight packages in order that the tellurite fiber amplifiers may be applied to generally optical communication systems.
Recently, in addition to the present 1530-1570 nm communication band, available wavelength bands in optical communication have been expanded to a 1570 -1610 nm band in order to greatly increase communication capacity. Then, communication devices to be used in the widened wavelength band are required to have good operations in longer wavelengths over the 1530-1570 nm band, and at the same time, broadband ASE light sources for testing the broadband communication devices are required to provide coverage of such a broader band.
However, the conventional single light source with ASE using quartz- or fluoride-based fibers can emit either the 1530-1570 nm band (1550 nm band) or the 1570-1610 nm band (1580 nm band) on the practical base, but no single ASE source for emitting both bands with flattened light levels has been developed. Two ASE light sources to emit each different band, 1550 nm band and 1580 nm band, can be mixed into a single fiber, but the application of two ASE sources and a multiplexer for mixing the two sources is costly and disadvantageous.
It could be considered that the tellurite-based ASE source noted above was used to spread the emitting light band, but it was difficult to deal with the tellurite-fiber for practical usage. Also, the tellurite-based ASE source can emit at the 1580 nm band the power only lower than a tenth times as high as at the 1550 nm band, and therefore, high power pumping light is required to increase the output power of the 1580 nm band.
Aspects of the present invention can provide an ASE light source being capable of emitting high power broadband light by using a low pumping power with low cost.
Aspects of the present invention can provide an ASE light source, particularly, using amplifying Er-doped fiber to spread the flattened emitting band to 1550 nm and 1580 nm bands with a low pumping light power.
A broadband ASE light source includes: an amplifying optical fiber doped with rare earth; a pumping light source and an output port both which are coupled to one end of the amplifying optical fiber via a multiplexer; and a reflecting or circulating member which is coupled to the other end of the amplifying optical fiber.
Preferably, the amplifying optical fiber may have so large a length as to provide amplified spontaneous emission (ASE) light having a broad wavelength band containing a first and second wavelength bands individual to the doped ions through the port.
The reflecting or circulating member in the ASE light source can reflect or circulate light from the amplifying fiber to return into the same amplifying fiber, and then increase and/or flatten the amplitude over the broad wavelength band.
The broadband ASE light source may further include a second pumping source which is connected to the other end of the amplifying optical fiber along with the reflecting or circulating member. In this ASE light source, the previous (i.e., first) pumping source and the second pumping source supply pumping light for the one end and the other end, respectively, of the amplifying optical fiber, to emit the broadband ASE light including a first and second wavelength bands wherein the second pumping light source compensates for the potentially low level of the first band to increase.
Further, the broadband ASE light source may include only the one pumping light source which supplies its pumping light for both ends of the amplifying optical fiber. In this type, a distributing member may be coupled between the pumping light source and both ends of the amplifying optical fiber via respective multiplexers in order to divide the pumping light for the one end and the other end of the amplifying optical fiber (i.e., for the one end of the amplifying fiber along with the output port, and for the other end of the same along with the reflecting or circulating member).
Particularly, the amplifying optical fiber may include an erbium-doped optical fiber. The amplifying optical fiber may be based on silica or fluoride.
By using an Er-doped optical fiber, the ASE light source amplifying can emit broadband light in a flattened spectral range of 1530 to 1610 nm, composed of the 1550 nm band (1530-1570 nm) and the 1580 nm band (1570-1610 nm), both wavelength bands individual in stimulated emission light of Er ions.
The broadband ASE light sources can be used as broadband light sources for detecting spectrum characteristics of optical devices, particularly, optical communication systems, for using wavelength division multiplexer (WDM) transmission techniques in the above broadband wavelength.